cpuidle: psci: Convert PM domain to platform driver

To enable support for deferred probing and to allow implementation of the
->sync_state() callback from subsequent changes, let's convert into a
platform driver.

Reviewed-by: Lina Iyer <ilina@codeaurora.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>